An extension of the sliding filament model for vertebrate skeletal muscle is the correlation between thick and thin filament lengths, the degree of filament overlap and the development of tension. The hypothesis for vertebrate striated muscle is based on sarcomeres with limited filament lengths. As far as we know, this hypothesis has not been tested on other muscles which have different filament lengths. The overall objective is to ascertain from length-tension curves and sarcomere lengths and filament length data whether various invertebrate and vertebrate striated muscles behave as predicted by the model. The specific aims are: 1. To determine the structure of sarcomeres in several striated muscles at various degrees of stretch; 2. To determine the length, diameter and periodicity of thick and thin filaments; 3. To determine the length-tension curves of these muscles; and 4. To correlate the length tension curves with average sarcomere lengths and thus with the sarcomeric organization. We will use the techniques of electron microscopy of sectioned material and negatively stained isolated filaments and optical diffraction of living, glycerinated and fixed muscle to determine the structure. Length-tension curves will be measured simultaneously with optical diffraction pattern determinations. Thus we will be able to correlate the length-tension curves with the sarcomeric structure.